Ag@TiO2 nanocomposite; Synthesis, characterization and its application as a novel and recyclable catalyst for the one-pot synthesis of benzoxazole derivatives in aqueous media

Article abstract of DOI:10.1039/c5ra06618b

In the present study, we use a Ag@TiO₂ nanocomposite for the efficient synthesis of benzoxazole derivatives via the one-pot condensation of 2-aminophenol and several aromatic aldehydes or orthoesters or carboxylic acids or amides or acyl chlorides in water at room temperature. In all situations the preferred products were synthesized with excellent yields. The short reaction times, high yields, safety and mild conditions, simplicity, non-toxicity and easy workup are the main merits of this protocol.

Full text of DOI:10.1039/c5ra06618b

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Graphical Abstract
Ag@TiO2
nanocomposite;
synthesis,
characterization and its application as a novel
and recyclable catalyst for the one-pot
synthesis of benzoxazole derivatives in
aqueous media
RCONH2
5
RCOOH
NH2
OH
Behrooz Maleki, Mehdi Baghayeri, Seyed
Mohammad Vahdat, Abbas Mohammadzadeh,
Somaieh Akhoondi
N
H2O
rt
4
RCHO
R
6
O
Ag@TiO2
1
7
RC(OR1)3
Ag@TiO2 nanocomposite/water as novel
catalytic system is used for the synthesis of
benzoxazole derivatives. Less reaction time
along with high product yield, catalyst stability
and recyclability are merits of this novel
protocol.
RCOCl
3
2

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Ag@TiO2 nanocomposite; synthesis, characterization and its
application as a novel and recyclable catalyst for the one-pot
synthesis of benzoxazole derivatives in aqueous media
Behrooz Maleki*a, Mehdi Baghayeria, Seyed Mohammad Vahdatb, Abbas
Mohammadzadehb, Somaieh Akhoondic
aDepartment of Chemistry, Hakim Sabzevari University, Sabzevar 96179ꢀ76487, Iran
*Eꢀmail:b.maleki@hsu.ac.ir, Tel:+985144013324
bDepartment of Chemistry, Islamic Azad University, Ayatollah Amoli Branch, P.O. Box 678,
Amol, Iran
cDepartment of Physics, Nour Branch, Islamic Azad University, Nour, Iran
Abstract:
In the present study, we use of Ag@TiO2 nanocomposite for the efficient synthesis of
benzoxazole derivatives via the oneꢀpot condensation of 2ꢀaminophenol and several aromatic
aldehydes or orthoesters or carboxylic acids or amides or acyl chlorides in water at room
temperature. In all situations the favorite product were synthesized with excellent yield. The
short reaction times, high yields, safety and mild conditions, simplicity, nonꢀtoxicity and easy
workup are the main merits of this protocol.
Keywords: Ag@TiO2 nanocomposite, Benzoxazole, 2ꢀAminophenol, Aldehydes, Orthoesters,
Carboxylic acids, Amides, Acyl chlorides, Green chemistry
Introduction
The benzoxazole moiety is created in a variety of biologically active natural compounds1 and
pharmaceutical agents.2ꢀ3 The benzoxazole derivatives are significant targets in drug discovery4
and also find applications in materials chemistry as photochromic agents,5 and as fluorescent
blenching agent dyes such as 1,4ꢀbis(benzoꢀxazolylꢀ2ꢀyl) naphthalene and arenes.6 Instead, the
1

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benzoxazole scaffold is found in a varied spectrum of biologically active compounds such as
antibiotic,7 anticancer agent,8 antimicrobial,9 antiꢀfungal,10 antiviral,11 gramꢀpositive antiꢀ
bacterials,12
antiparꢀkinson,13
antiꢀinflammatory,14
antitumor,15
antiꢀconvulsant,13ꢀ16
immunosuppressive agent,17 antiꢀparasitic,18 elastase inhibitor.19 Further important significant
physiological activities related with benzoxazoles are HIV reverse transcriptase inhibitor Lꢀ
697,661,20 H2ꢀantagonists,21 5HT3 receptor antagonist,22 selective peroxisome proliferatorꢀ
activated receptor γ antagonist JTPꢀ 426467,23 cytotoxicity towards P338 cells.24
Because of the importance of benzoxazoles in the pharmaceutical, dye, and photo industries,
many synthetic methods for these compounds have been widely reported. The most traditional
method for the synthesis of the benzoxazole has been included the condensation of 2ꢀ
aminophenols with aldehydes,25 orthoesters,26 carboxylic acids,27 amides,28 acyl cholorides.29
Nevertheless, most of these methods suffer from one or more of the subsequent drawbacks such
as low yields of the products, the use of expensive, toxic or nonꢀreusable catalysts, long reaction
times, tedious workꢀup procedures, high temperatures, harsh reaction conditions such as
requirement of strong acidic conditions, need to use extra amounts of reagent, the use of large
amounts of toxic and hazardous solvents such as xylene and pyridine and coꢀoccurrence of several
side reactions. Hence, it is required to find a better catalyst for the synthesis of benzoxazoles in
terms of working simplicity, nonꢀtoxicity, reusability, environmentally friendly and economical
acceptability.
Results and discussions
As a part of continuing efforts in our laboratory towards the synthesis of various organic
compounds and the use of nanoparticles in new synthetic methodology,30 we reported on the
synthesis benzoxazoles using Ag@TiO2 nanocomposite as the catalyst. Preparing ultrathin nanoꢀ
sized noble metal particles, particularly with diameters less than 5 nm and even less than 1 nm, is
an efficient way to increase their catalytic activity while reducing the amount of noble metal
used.31 To improve photocatalytic activity, noble metals such as Ag, Au, Pt and Pd are deposited
on a TiO2 surface, because they act as an electron trap promoting interfacial charge transfer
processes in the composites. However this type of catalyst structure is effective, metals on the
surface of the semiconductor are easily oxidized and dissolved.32 Ag@TiO2 nanocomposite, in
individual, are drawing much attention as catalysts in organic syntheses on account of their easy
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preparation, physical and chemical stability, ease of recyclability, high oxidative influence, and
low cost of production as compared to other catalysts.33
In this exploration, the synthesis of benzoxazole derivatives (7) from the condensation of
2‐aminophenol (6) with various aromatic aldehydes, orthoesters, carboxylic acids, amides and
acyl chlorides (1-5) in the presence of Ag (1.5%)@TiO2 as heterogeneous and reusable
nanocatalyst, in water at room temperature were studied (Scheme 1).
RCONH2
5
RCOOH
NH2
OH
N
O
H2O
rt
4
RCHO
R
6
Ag@TiO2
1
7
RC(OR1)3
RCOCl
3
2
Scheme 1 Synthesis of benzoxazole derivatives using nano Ag@TiO2
Recently, the development of environmentally benign and clean synthetic procedures has become
the goal of organic synthesis. Water plays an essential role in life processes and also a medium for
organic synthesis. The replacement of hazardous solvent with those that are environmentally benign is
an active area of current research. Water is clean, nonꢀtoxic, and hazardꢀfree in handling, nonꢀ
inflammable, cheap and a readily available solvent. Therefore, it is important to carry out organic
reactions in water instead of other solvents for environmental and economic reasons. Furthermore,
because of its highly polarity, high surface tension, high specific heat capacity and network of
hydrogen bonds, water plays a significant role in some reactions.34
Wellꢀdispersed Ag@TiO2 nanocomposite particles can be synthesized on a large scale by a clean
photochemical route which does not require any additives.35 50 mg of nano TiO2 powder and 1 mL (5
mol/L AgNO3) of an aqueous metal salt solution were dispersed with 100 mL of deionized water in a
Pyrex flask with capacity of about 250 mL under stirring. The flask was exposed to light from a highꢀ
3

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pressure Xe lamp (150 W). The reaction was carried for 25 min. Then, the powder was collected by
centrifugation and washed twice by deionized water and ethanol. Finally, the resulting powder was
dried at 60°C. Ag@TiO2 nanocomposites dispersion in water was used as such for the reactions.
Ag@TiO2 nanocomposite was characterized using atomic force microscopy (AFM), Xꢀray
diffraction (XRD), scanning electron microscope (SEM) and energy dispersive Xꢀray micro analysis
(EDX).
TiO2 and Ag@TiO2 were characterized by the SEM to determine the morphology of TiO2 and
Ag@TiO2 nanoparticles (Fig. 1).
A
B
Fig. 1 SEM micrograph of: (A) pure TiO2 and (B) Ag (1.5 %)@TiO2 nanoparticles.
Fig. 2(a) shows two dimensional AFM image and Fig. 2(b) shows three dimensional AFM image of
mentioned nano layer. The particle diameter was measured at different points of the surface by particle
analysis tool (Fig. 2b) and revealed that the surface roughness was determined as equal to about 1.8 nm
and the average size calculated was about 24 nm. In the line profile shown in (Fig. 2a) a similar surface
roughness of 1.7 nm was observed for Ag@TiO2.
The morphology of product was studied by tapping mode AFM. The atomic force microscopy
(AFM) analysis indicated that the surface roughness was increased by the addition of benzoxazole to
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the Ag@TiO2. Fig. 3 shows the AFM images of benzoxazoles deposited on Ag@TiO2 substrates and
indicated that the surface roughness was determined as equal to about 13.5 nm and the average size
calculated was about 65 nm (Fig. 3b). In the line profile shown in (Fig. 3a) a similar surface roughness
of 28.3 nm was observed for Ag@TiO2. The increased thickness is attributed to the benzoxazole
compounds on the Ag@TiO2.
(a)
(b)
Fig. 2 Twoꢀdimensional (a) and threeꢀdimensional (b) AFM images of the Ag@TiO2 composite particles
synthesized.
(a)
(b)
Fig. 3 Twoꢀdimensional (a) and threeꢀdimensional (b) AFM images of the 2ꢀ(2ꢀhydroxyphenyl)benzoxazole
deposited on Ag@TiO2.
Fig. 4 shows the XRD pattern of pure and silverꢀmodified TiO2 powders by the optimum amounts
of Ag prepared.
5

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Fig. 4 XRD patterns of pure (a) TiO2, (b) Ag (1%)@TiO2, (c) Ag (1.5%)@TiO2 and (d) Ag
(2%)@TiO2 nanoparticles.
Fig. 5 shows the EDX spectrum of synthesized nanoparticles. The spectrum shows the
characteristic peaks for both Ti and Ag confirming the formation of nanoparticles.
Fig. 5 EDX analysis of Ag (1.5%)@TiO2 nanoparticles.
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In order to optimize the reaction conditions, 2ꢀ aminophenol (1 mmol) and benzaldehyde (1 mmol)
were selected as the model reaction in the presence of Ag@TiO2. Different molar ratios of the catalyst,
different solvents, and various temperatures were examined in the model reaction (Table 1, Entries 1ꢀ
8). The best result was obtained in the presence of catalytic amount of Ag (1.5%)@TiO2
nanocomposite (0.1 ml) in water at room temperature, and 2ꢀphenylbenzoxazole was obtained in 92%
yield after 5 min ( Entry 2).
As shown in Table 1, when using catalytic amount of Ag (1.5%)@TiO2, the reaction gave
benzoxazoles with 92% yield in 5 min in water (entry 2), and further lowering the catalyst loading up to
1% led to lower yield of 82% in 10 min (entry 1). In the presence of Ag (2%)@TiO2 catalyst the
reaction affords the corresponding synthesis of 2ꢀphenylbenzoxazole in 92% yield within 5 min (entry
3).
To investigate the effect of solvent in these reactions, the reactions were done in different solvents.
The results are reported in (Table 1, entries 4ꢀ7). The best result is shown in water at room temperature.
To indicate the need of Ag (1.5%)@TiO2 for this condensation, we observed that the model reaction
did not proceed in the absence of Ag (1.5%)@TiO2 even after 12 h (entry 8).
Table 1 Optimization of reaction conditions for the synthesis of 2– (phenyl)ꢀbenzoxazole
Entry
Catalyst
Conditions
Time (min)
Yielda (%)
1
2
3
4
5
6
7
Ag (1%)@TiO2 (0.1 ml)
Ag (1.5%)@TiO2 (0.1 ml)
Ag (2%)@TiO2 (0.1 ml)
Ag (1.5%)@TiO2 (0.1 ml)
Ag (1.5%)@TiO2 (0.1 ml)
Ag (1.5%)@TiO2 (0.1 ml)
Ag (1.5%)@TiO2 (0.1 ml)
H2O/rt
H2O/rt
10
5
82
92
92
86
82
74
80
H2O/rt
5
Toluene/rt
Ethyl acetate/rt
CH2Cl2/rt
CH3CN/rt
20
15
10
15
7

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8
ꢀb
H2O/rt
12 (h)
ꢀꢀc
aIsolated yields. bIn the absence of Ag (1.5%)@TiO2, cNo reaction
Having established the optimized reaction conditions, we turned over attention to explore the
scope of this protocol. The results are listed in Table 2. As shown in Table 2, 2ꢀamino phenol
reacted with a wide variety of substituted benzaldehydes, orthoesters, carboxylic acids, amides
and acyl chlorides completely and afforded the corresponding benzoxazoles in good to excellent
yields under mild reaction conditions in water at room temperature. The reaction proceeds
smoothly at room temperature in moderate to good yields. An analysis of the effect of the
substituent in the reaction time and yields showed that as the electronꢀdonating power of the
substituent in the benzaldehydes, orthoesters, carboxylic acids, amides and acyl chlorides
increased, the reaction time was longer and the yield diminished (Table 2, 2, 8, 17, 19, 24). Also,
all the reactions were free from byꢀproducts usually found in classical reaction conditions. The
total disappearance of the starting material was checked by thinꢀlayer chromatography (TLC).
The products were fully identified from their melting points and spectral data, which were in full
agreement with the values described in the literature.
Table 2 Preparation of benzoxazole derivatives using Ag (1.5%)@TiO2 (0.1 ml) as catalyst
Entry Compounds
Benzoxazoles (7)
R
Time
Yielda
(%)
92
Mp (0C)
Found
(min)
(1-5)
Reported
1
2
3
4
5
6
C6H5CHO
C6H5
5
101ꢀ102 100ꢀ102 (ref. 25c)
114ꢀ116 112ꢀ113 (ref. 25c)
4ꢀCH3C6H4CHO
4ꢀCH3OC6H4CHO
4ꢀNO2C6H4CHO
4ꢀCNC6H4CHO
4ꢀClC6H4CHO
4ꢀCH3C6H4
4ꢀCH3OC6H4
4ꢀNO2C6H4
4ꢀCNC6H4
4ꢀClC6H4
10
8
91
92
100ꢀ102
264ꢀ266
200ꢀ203
148ꢀ150
97ꢀ99 (ref. 29e)
2
96
264ꢀ265 (ref. 25c)
200ꢀ202 (ref. 25h)
147ꢀ149 (ref. 25g)
2
94
5
90
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7
4ꢀBrC6H4CHO
2ꢀHOC6H4CHO
2ꢀClC6H4CHO
3ꢀNO2C6H4CHO
2ꢀNO2C6H4CHO
C6H5COCl
4ꢀBrC6H4
2ꢀHOC6H4
2ꢀClC6H4
3ꢀNO2C6H4
2ꢀNO2C6H4
C6H5
5
91
88
90
93
90
93
92
90
93
90
87
84
82
90
89
90
86
82
84
156ꢀ157
122ꢀ124
70ꢀ72
211ꢀ213
100ꢀ102
101ꢀ102
148ꢀ150
71ꢀ72
Oil
156ꢀ157(ref. 25g)
122ꢀ124(ref. 25g)
70ꢀ73 (ref. 29e)
211ꢀ212 (ref. 25c)
101ꢀ102 (ref. 25c)
103ꢀ104 (ref. 29f)
149ꢀ151 (ref. 29f)
70ꢀ73 (ref. 25i)
Oil (ref. 26c)
8
10
10
4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
5
4
4ꢀClC6H5COCl
3ꢀCH3OC6H5COCl
HC(OCH3)3
4ꢀClC6H5
3ꢀCH3OC6H5
H
4
6
4
HC(OCH2CH3)3
CH3C(OCH3)3
CH3C(OCH2CH3)3
H
5
Oil
Oil (ref. 26c)
CH3
7
Oil
Oil (ref. 26c)
CH3
8
Oil
Oil (ref. 26c)
CH3CH2C(OCH2CH3)3 CH3CH2
10
10
8
Oil
Oil (ref. 26c)
C6H5COOH
C6H5
101ꢀ102
148ꢀ150
71ꢀ72
101ꢀ102
Oil
102 (ref. 27d)
4ꢀClC6H5COOH
3ꢀCH3OC6H5COOH
C6H5CONH2
CH3CONH2
4ꢀClC6H5
3ꢀCH3OC6H5
C6H5
151ꢀ152 (ref. 29f)
70ꢀ72 (ref. 27d)
99ꢀ100 (ref. 28a)
Oil (ref. 28a)
12
15
20
15
CH3
HCONH2
H
Oil
Oil (ref. 28a)
aIsolated yields.
A possible mechanism of Ag@TiO2ꢀcatalyzed reaction of 2ꢀaminophenol with aldehydes or
orthoesters or carboxylic acids or amides or acyl chlorides for the synthesis of benzoxazole is
shown in Scheme 2.25ꢀ29 The catalyst Ag@TiO2 would presumably first coordinate with oxygen
group in aldehydes or orthoesters or carboxylic acids or amides or acyl chlorides. Then,
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intermediates A-C were formed through the reaction of amino group with activated aldehydes or
orthoesters or carboxylic acids or amides or acyl chlorides under the present reaction conditions.
Subsequently, hydroxyl groups in A-C attacked the carbonyl or imine groups to accomplish
intermolecular addition/cyclization and generated intermediate product D-E, which followed by
dehydration to form the desired benzoxazole product 7.
OH
NH
R
H2N
OR1
OR1
O
R
NH2
OH
OH
R
O
O
OR1
R1
NH
R
A
O
H
R
C
OR1
O
N
O
R
OH
R
OH
Cl
R
B
= Ag@TiO2
R
OR1
O
N
O
R
O
NH
HN
O
R
7
H
D
E
Scheme 2 Plausible mechanism.
The reusability of the catalyst is an important factor from economic and environmental point of
views and has attracted much attention in recent years. Therefore, the recovery and reusability of
Ag@TiO2 nanocomposite was investigated in the reaction of benzaldehyde with 2ꢀaminophenol
under optimized reaction conditions. The catalyst could be recovered simply by solvent extraction of
the product from the reaction mixture using ethyl acetate. The recovered catalyst was dried at 60oC
and reused. The results showed that the catalyst can be used 4 times without loss of its activity (Fig.
6).
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Fig. 6 Reusability of Ag (1.5%)@TiO2 nanocomposite for model reaction.
Conclusions
In conclusion, we have reported an effective, simple, inexpensive and green method for the
synthesis of benzoxazoles by condensation between 2ꢀamino phenols with various aldehydes or
orthoesters or carboxylic acids or amides or acyl chlorides in water at room temperature. The use of
Ag@TiO2 nanocomposite dispersed in water as a highly efficient, inexpensive and reusable catalyst
makes the present procedure environmentally and economically acceptable. In addition, short
reaction times, excellent yields of product, least catalyst loading, safety and mild reaction conditions
and easy workꢀup procedure are other noteworthy advantages which make this method a valid
contribution to the existing process in the field of these heterocyclic synthesis.
Experimental
All the chemicals and solvents used were of research grade reagents and were used as received
from Merck and SigmaꢀAldrich Co. without further purification. Titanium oxide (nano TiO2, size
∼21 nm), silver nitrate (AgNO3, 99%), 2ꢀaminophenol, aldehydes, orthoester, carboxylic acids,
amides, and acyl chlorides from SigmaꢀAldrich (United States); EtOH from Merck (China); in
commercial grade. Deꢀionized water obtained from a PURE ROUP 30 water purification system
was used in these experiments. The melting points of synthesized compounds were determined
1
by open capillary method and were uncorrected. All H NMR and 13C NMR spectra were
recorded in DMSO on a FTꢀNMR Bruker DRX (1H NMR, 400 MHz; 13C NMR, 100 MHz). All
Proton chemical shifts (ꢀ) are relative to tetramethylsilane (TMS, δ = 0.00) as an internal
standard and coupling constants (J) are given in Hz. The IR spectra were recorded on a Rayleigh
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WQFꢀ510 spectrometer using KBr discs. The particles size of Ag@TiO2 and roughness of the
films were evaluated by atomic force microscopy (AFM Easyscan 2 Flex). The crystalline phase
of nanoparticles was analyzed by XRD measurements by using Rigaku D/Maxꢀ2200 model
diffract meter equipped with horizontal goniometric in θ/2θ geometry. Morphological studies
were conducted using a SEM (SEM, Viga II, 3×105, USA). The purity and homogeneity of
compounds were checked using TLC technique. All products are known compounds and were
characterized by comparison of their spectral and physical data with authentic samples prepared
according to the literature methods.
General Procedure for the Synthesis of Benzoxazole Derivatives
A mixture of 2ꢀaminophenol (1 mmol), aldehyde or orthoester or carboxylic acid or amide or acyl
chloride (1 mmol), and Ag (1.5%)@TiO2 nanocomposites dispersion in water (0.1 ml) in 5 ml of
water [for insoluble aldehydes 0.2 ml of ethanol was added] was stirred at room temperature for an
appropriate time (Table 2). The progress of the reaction was monitored by TLC using nꢀHexane:
EtOAc (5:2). After complete conversion, EtOAc was added to the mixture and the product was
extracted into EtOAc, the organic phase was washed with H2O and dried with MgSO4. Evaporation
of the solvent gave the crude product. The product recrystallized from ethanol (96%, 2 ml) to get
pure benzoxazole derivatives. All products were identified by comparison of their physical and
spectroscopic data with those reported for authentic samples.
Acknowledgments
This research work was supported by the the University of Hakim Sabzevari and Islamic Azad.
Authors wish to thank the University of Hakim Sabzevari and Islamic Azad for financial support to
carry out this research.
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